1. Field of the Invention
The invention relates to a display apparatus using capacitive light emitting devices such as organic electroluminescence devices or the like and its driving method.
2. Description of the Related Art
As a display in which an electric power consumption is low and a display quality is high and, further, a thin size can be realized, an electroluminescence display constructed by arranging a plurality of organic electroluminescence devices in a matrix shape is highlighted. As shown in FIG. 1, the organic electroluminescence device is constructed in a manner such that an organic function layer 102 of at least one layer consisting of an electron transporting layer, a light emitting layer, a hole transporting layer, or the like and a metal electrode 103 are laminated on a transparent substrate 100 made of a glass substrate or the like on which a transparent electrode 101 is formed. When a plus voltage is applied to an anode of the transparent electrode 101 and a minus voltage is applied to a cathode of the metal electrode 103, namely, when a direct current is applied across the transparent electrode and the metal electrode, the organic function layer 102 emits light. An organic compound in which good light emitting characteristics can be expected is used as an organic function layer, thereby enabling the electroluminescence display to endure a practical use.
The organic electroluminescence device (hereinafter, simply referred to as a device) can be electrically expressed by an equivalent circuit as shown in FIG. 2. As will be understood from the diagram, the device can be replaced with a construction comprising a capacitance component C and a component E of characteristics of a diode connected in parallel to the capacitance component. The organic electroluminescence device is, therefore, regarded as a capacitive light emitting device. According to the organic electroluminescence device, when a DC light emission driving voltage is applied across the electrodes, charges are stored in the capacitance component C. Subsequently, when the applied voltage exceeds a barrier voltage or a light emission threshold voltage that is peculiar to the device, a current starts on flowing to the organic function layer serving as a light emitting layer from the electrode (anode side of the diode component E) and the device emits the light at an intensity which is proportional to the current.
The characteristics of a voltage Vxe2x80x94a current Ixe2x80x94a luminance L of the device are similar to those of the diode as shown in FIG. 3. When the device is supplied with a voltage of a light emission threshold value Vth or less, the current I is extremely small. When the voltage exceeds the light emission threshold value Vth, the current I suddenly increases. The current I is almost proportional to the luminance L. According to the device, if a driving voltage exceeding the light emission threshold value Vth is applied to the device, the light emission luminance proportional to the current according to the driving voltage is provided. If the driving voltage applied is equal to or less than the light emission threshold value Vth, no driving current flows and the light emission luminance is equal to zero.
A simple matrix driving system can be applied as a driving method of a display panel using a plurality of organic electroluminescence devices. FIG. 4 shows a structure of an example of a simple matrix display panel. n cathode lines (metal electrodes) B1 to Bn are extended and provided in parallel in the lateral direction and m anode lines (transparent electrodes) A1 to Am are extended and provided in parallel in the vertical direction. Light emitting layers of organic electroluminescence devices E1,1 to Em,n are sandwiched in (total nxc3x97m) crossing portions of the cathode lines and the anode lines. The devices E1,1 to Em,n serving as pixels are arranged in a lattice shape. In correspondence to each crossing position of the anode lines A1 to Am in the vertical direction and the cathode lines B1 to Bn in the horizontal direction, one end (anode line side of the diode component E of the equivalent circuit) is connected to the anode line and the other end (cathode line side of the diode component E of the equivalent circuit) is connected to the cathode line. The cathode lines are connected to a cathode line scanning circuit 1. The anode lines are connected to an anode line driving circuit 2.
The cathode line scanning circuit 1 has scan switches 51 to 5n corresponding to the cathode lines B1 to Bn in which an electric potential of each cathode line is individually determined. Each scan switch applies either an inverse bias potential Vcc (for example, 10V) which is obtained from a power voltage or a ground potential (0V) to the corresponding cathode line.
The anode line driving circuit 2 has current sources 21 to 2m (for example, constant current sources) and drive switches 61 to 6m corresponding to the anode lines A1 to Am for individually supplying a driving current to each device through each anode line and is constructed in a manner such that the drive switch is on/off controlled so as to individually supply a current to each anode line. A voltage source such as a constant voltage source can be also used as a driving source. A current source (power supplying circuit whose supply current amount is controlled so as to have a desired value) is generally used because of reasons such that voltagexe2x80x94luminance characteristics are unstable for a temperature change although the currentxe2x80x94luminance characteristics are stable for a temperature change and the like. The supply current amount of each of the current sources 21 to 2m is set to a current amount that is necessary to maintain a state where the device emits the light at a desired instantaneous luminance (hereinafter, the state is referred to as a stationary light emitting state). When the device is in the stationary light emitting state, the charges corresponding to the supply current amount are stored in the capacitance component C of the device. Thus, a voltage across the device is equal to a specified value Ve (hereinafter, referred to as a specified light emission voltage) corresponding to the instantaneous luminance.
The anode lines are also connected to an anode line resetting circuit 3. The anode line resetting circuit 3 has shunt switches 71 to 7m provided every anode line. When the shunt switch is selected, the corresponding anode line is set to a ground potential.
The cathode line scanning circuit 1, anode line driving circuit 2, and anode line resetting circuit 3 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1, anode line driving circuit 2, and anode line resetting circuit 3 in accordance with image data supplied from an image data generating system (not shown) so as to display an image shown by the image data. The light emission control circuit 4 generates a scanning line selection control signal to the cathode line scanning circuit 1 and controls so as to switch the scan switches 51 to 5n in a manner such that one of the cathode lines corresponding to a horizontal scanning period of the image data is selected and set to the ground potential and the inverse bias potential Vcc is applied to the other cathode lines. The inverse bias potential Vcc is applied by the constant voltage source connected to the cathode line in order to prevent that the device connected to the crossing point of the anode line which is at present being driven and the cathode line in which a scan selection is not performed emits light due to crosstalk. The inverse bias potential Vcc is generally set so that Vcc=specified light emission voltage Ve. Since the scan switches 51 to 5n are sequentially switched to the ground potential every horizontal scanning period, the cathode line set to the ground potential functions as a scanning line which enables the device connected to the cathode line to emit light.
The anode line driving circuit 2 executes a light emission control to the scanning line. The light emission control circuit 4 generates a drive control signal (driving pulse) showing which one of the devices connected to the scanning line is allowed to emit light at which timing and how long in accordance with the pixel information shown by the image data and supplies it to the anode line driving circuit 2. In response to the drive control signal, the anode line driving circuit 2 on/off controls some of the drive switches 61 to 6m and supplies a driving current to the devices according to the pixel information through the anode lines A1 to Am. The device to which the driving current was supplied emits light in accordance with the pixel information.
The resetting operation of the anode line resetting circuit 3 is executed in response to the reset control signal from the light emission control circuit 4. The anode line resetting circuit 3 turns on any switches of the shunt switches 71 to 7m corresponding to the anode line as a reset target shown by the reset control signal and turns off the other shunt switches.
A driving method of performing the resetting operation to discharge the charges stored in each device arranged in the lattice shape just before the scanning lines in the simple matrix display panel (hereinafter, referred to as a reset driving method) as disclosed in JP-A-9-232074 filed by the same applicant as that of the present invention. The reset driving method intends to makes a timing for activating the light emission of the device when the scanning line is switched early. The reset driving method of the simple matrix display panel will be described with reference to FIGS. 4 to 6.
The operations shown in FIGS. 4 to 6, which will be explained hereinlater, relates to the case where after the cathode line B1 was scanned and the devices E1,1 and E2,1 were lit on, the cathode line B2 is scanned and the devices E2,2 and E3,2 are lit on as an example. For making the description easy, the lit-on device is shown by a diode symbol and the lit-off light emitting device is shown by a capacitor symbol. The inverse bias potential Vcc which is applied to the cathode lines B1 to Bn is set to 10V that is the same as the specified light emission voltage Ve of the device.
First, in FIG. 4, only the scan switch 51 is switched to the ground potential side of 0V and the cathode line B1 is scanned. The inverse bias potential Vcc is applied to the other cathode lines B2 to Bn by the scan switches 52 to 5n. At the same time, the current sources 21 and 22 are connected to the anode lines A1 and A2 by the drive switches 61 and 62. The other anode lines A3 to Am are switched to the ground potential side of 0V by the shunt switches 73 to 7m. In the case of FIG. 4, therefore, only the devices E1,1 and E2,1 are biased in the forward direction, the driving current flows from the current sources 21 and 22 as shown by arrows, and only the devices E1,1 and E2,1 perform the light emission. In the state, each of the non-light emitting devices E3,2 to Em,n shown by hatched capacitor symbols is charged to a polarity as shown in the diagram.
The following reset control is performed just before the scan is shifted from the stationary light emitting state shown in FIG. 4 to a next state where the devices E2,2 and E3,2 perform the light emission. That is, as shown in FIG. 5, all of the drive switches 61 to 6m are turned off, all of the scan switches 51 to 5n and all of the shunt switches 71 to 7m are switched to the ground potential side of 0V, and all of the anode lines A1 to Am and the cathode lines B1 to Bn are once shunted to the ground potential side of 0V, thereby all-resetting them. When the all-resetting operation is performed, since all of the anode lines and the cathode lines are set to the same electric potential of 0V, the charges stored in each device are discharged along routes shown by arrows in the diagram and the stored charges of all devices are instantaneously extinguished.
After the stored charges of all devices are set to zero as mentioned above, only the scan switch 52 corresponding to the cathode line B2 is subsequently switched to the 0V side as shown in FIG. 6 and the cathode line B2 is scanned. At the same time, the drive switches 62 and 63 are closed, the current sources 22 and 23 are connected to the corresponding anode lines, the shunt switches 71 and 74 to 7m are turned on, and 0V is applied to the anode lines A1 and A4 to Am.
As mentioned above, in the light emission control of the reset driving method, a scan mode serving as a period of time during which one of the cathode lines B1 to Bn is made active and a subsequent reset mode are repeated. The scan mode and the reset mode are executed every horizontal scanning period (1H) of the image data. Now, assuming that the control mode is directly shifted from the state of FIG. 4 to the state of FIG. 6 without performing the reset control, for example, the driving current which is supplied from the current source 23 not only flows into the device E3,2 but also is expended to cancel the reverse direction charges (shown in FIG. 4) stored in the devices E3,3 to E3,n, so that it takes time to set the device E3,2 into the stationary light emitting state (the voltage across the device E3,2 to the specified light emission voltage Ve).
When the reset control is performed, however, since the potentials of the anode lines A2 and A3 are set to approximately Vcc at a moment when the scan is switched to the scan of the cathode line B2, the charging currents are supplied to the devices E2,2 and E3,2 to be subsequently lit on from not only the current sources 22 and 23 but also a plurality of routes from the constant voltage sources connected to the cathode lines B1 and B3 to Bn. A parasitic capacitance is charged by the charging currents, the voltage instantaneously reaches the specified light emission voltage Ve and the device can be instantaneously shifted to the stationary light emitting state. After that, within the scanning period of the cathode line B2, since an amount of current which is supplied from the current source as mentioned above is set to a current amount such that device can maintain the stationary light emitting state at the specified light emission voltage Ve, the currents which are supplied from the current sources 22 and 23 flow into only the devices E2,2 and E3,2 and all of them are expended for light emission. That is, the light emitting state shown in FIG. 6 is continued.
According to the conventional reset driving method as mentioned above, since all of the cathode lines and anode lines are once connected to 0V as a ground potential or the same electric potential of the inverse bias potential Vcc and reset before the control mode is shifted to the light emission control of the next scanning line, when the scanning line is switched to the next scanning line, the charging time until the specified light emission voltage Ve is shortened and the activating speed of the light emission of the device to perform the light emission on the switched scanning line can be made fast.
The voltage levels of the cathode lines and anode lines in the operations shown in FIGS. 4 to 6 are shown by a timing chart of FIG. 7. In a scanning period j, the voltage across each of the devices existing on the crossing points of the cathode line B1 and anode lines A1 and A2 is set to an anode line voltage level VAA (equal to Ve in FIGS. 4 to 6) and the light emission is performed at the luminance corresponding to the anode line voltage level VAA. In a next scanning period j+1, the voltage across each of the devices existing on the crossing points of the cathode line B2 and anode lines A2 and A3 is set to an anode line voltage level VAA (equal to Ve in FIGS. 4 to 6) and the light emission is performed at the luminance corresponding to the anode line voltage level VAA.
In the light emission display using the conventional reset driving method mentioned above, in the case of performing the luminance adjustment, a general luminance adjusting method of the matrix display is applied. That is, as shown in FIG. 7 there is a method whereby the level of the voltage across the device at the time of the light emission is set to a constant value (that is, the constant instantaneous luminance and constant driving current of the device) and a connecting time of a driving source to the anode line is changed within a range of the scanning period of time, thereby adjusting the light emission luminance of each device (pulse width modulating method).
That is, if a luminance gradation (dimmer) is applied in dependence on a length of the driving time within each scanning period, the scanning period j in FIG. 7 relates to the case of the dimmer of 100% in which the luminance is the maximum because the light emitting state is continued until the end of the period. The scanning period j+1 relates to the case of the dimmer of 50% because the light emitting state is continued until the time point of the half of the period. The scanning period j+2 relates to the case of the dimmer of 80% because the light emitting state is continued until the time point of 80% of the period.
Within the scanning period in the cases other than the dimmer of 100%, the grounding operation is performed as shown in FIG. 8 until the period is shifted to the resetting period after completion of the operation corresponding to a dimmer percentage shown in FIG. 4. That is, the drive switches 61 and 62 are turned off and, at the same time, all of the shunt switches 71 to 7m are switched to the ground potential side of 0V. All of the anode lines A1 to Am, consequently, are set to the ground potential. Since the cathode line B1 is held to the ground potential, the charges stored in the devices E1,1 and E2,1 are discharged along routes as shown by arrows in the diagram. The stored charges of the devices E1,1 and E2,1 are instantaneously extinguished. Since the cathode lines B2 to Bn are held in a state where the inverse bias potential Vcc was applied by the scan switches 52 to 5n in the state, each of the non-light emitting devices E1,2 to E1,n E2,2 to E2,n, . . . , and Em,2 to Em,n shown as hatched capacitor symbols in FIG. 8 is charged to a polarity as shown in the diagram or maintains the charging state by the polarity. When the resetting period comes after that, the operation shown in FIG. 5 is executed.
In the B1 scanning period of the dimmer of 100%, as shown in FIG. 9A after the cathode line B1 was scanned and the devices E1,1 and E2,1 were lit on, in the case where the scan is shifted to the cathode line B2 in the resetting period and the next B2 scanning period and the devices E2,2 and E3,2 are lit on, the emission of the charges of [2+(mxe2x88x922)(nxe2x88x921)]e occurs in the resetting period. The charging of the charges of [2+(mxe2x88x922)(nxe2x88x921)]e is performed in the B2 scanning period. Now, assuming that m=4 for easy understanding, an emission amount of the charges in the resetting period is equal to 2ne and a charging amount of the charges in the B2 scanning period is equal to 2ne.
In the B1 scanning period of the dimmer of 50%, as shown in FIG. 9B the light emitting operation for scanning the cathode line B1 and allowing the devices E1,1 and E2,1 to be lit on and the grounding operation to connect them to the ground by the shunt switches 71 to 7m as mentioned above are sequentially performed by an amount of 50% at a time. After that, the scan is shifted to the cathode line B2 in the resetting period and the next B2 scanning period and the devices E2,2 and E3,2 are lit on. In the case, the emission of the charges of 2e and the charging of the charges of (mxe2x88x922)(nxe2x88x921)e are performed in the grounding operation within the B1 scanning period. The emission of the charges of (mxe2x88x922)(nxe2x88x921)e occurs in the resetting period. The charging of the charges of [2+(mxe2x88x922)(nxe2x88x921)]e is similarly performed in the B2 scanning period. Now, assuming that m=4, a charging amount of the charges in the grounding operation in the B1 scanning period is equal to 2(nxe2x88x921)e, an emission amount of the charges in the resetting period is equal to 2(nxe2x88x921)e and a charging amount of the charges in the B2 scanning period is equal to 2ne.
There is a problem such that in the case of getting the intermediate luminance in which the grounding operation is included in the scanning period like the case of the dimmer of 50%, an invalid electric power consumption is larger than that in the case of the maximum luminance of the dimmer of 100%.
It is, therefore, an object of the invention to provide a display apparatus of capacitive light emitting devices which can reduce an electric power consumption in the case of a gradation display and an intermediate luminance and to provide its driving method.
According to the invention, there is provided a driving method of a display apparatus having a plurality of driving lines and a plurality of scanning lines and a plurality of capacitive light emitting devices connected between the scanning lines and the driving lines at a plurality of crossing positions by the driving lines and the scanning lines, comprising the steps of: selecting some of the plurality of driving lines for a scanning period in a predetermined cyclic period consisting of the scanning period and a resetting period subsequent thereto, sequentially selecting one of the plurality of scanning lines, connecting current sources to the selected driving lines, and supplying a current in the forward direction to each of the capacitive light emitting devices between the selected driving lines and the selected one scanning line; and in the resetting period, applying a same electric potential to driving lines to be selected for at least a next scanning period and all of the plurality of scanning lines, thereby discharging charges of the capacitive light emitting devices between the driving lines to be selected and all of the scanning lines, wherein a length of the scanning period in the predetermined cyclic period is changed in response to a luminance information command indicative of a display luminance, and the period other than the scanning period in the predetermined cyclic period is set to the resetting period.
According to the invention, there is provided a display apparatus comprising: a plurality of driving lines and a plurality of scanning lines; a plurality of capacitive light emitting devices connected between the scanning lines and the driving lines at a plurality of crossing positions by the driving lines and the scanning lines; scanning period control means for selecting some of the plurality of driving lines in a scanning period in a predetermined cyclic period consisting of the scanning period and a resetting period subsequent thereto, sequentially selecting one of the plurality of scanning lines, connecting current sources to the selected driving lines, and supplying a current in the forward direction to each of the capacitive light emitting devices between the driving lines to be selected and the selected one scanning line; and resetting period control means for applying a same electric potential to driving lines to be selected for at least a next scanning period and all of the plurality of scanning lines in the resetting period, thereby discharging charges of the capacitive light emitting devices between the driving lines to be selected and all of the scanning lines, wherein a length of the scanning period in the predetermined cyclic period is changed in response to a luminance information command indicative of a display luminance, and the period other than the scanning period in the predetermined cyclic period is set to the resetting period.